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Piston Pilot Problems

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Piston Pilot Problems

Unread postAuthor: thedeathofall » Fri Jan 23, 2009 11:47 pm

Hey guys,
I'm not new to the forum but this happens to be my first time posting. I have about 3 years of cannon experience, but there come a time when a guy just needs some help.

I've been working on a small/medium scale, 1 inch, Tee, piston rifle. The piston is made of solid pvc rod with neoprene to seal. It slides and seals nicely, however, it wont let the air through to the barrel. I think i might need a different pilot valve, but that is why I'm asking the experts. The barrel is 1/2 inch SECH40 pvc. (The barrel looks kind of bent in the pics, but it isn't.)

The point of this gun is to make a small scale weapon to use in airsoft/paintball. I have created a great airsoft shell that costs almost nothing to make and I am considering cutting down the barrel to make it more of a bolt-action, saw'd off shotgun. If all goes well I will eventually include a backpack C02 rig.
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Attachments
piston resize.jpg
The piston...
piston resize.jpg (18.48 KiB) Viewed 471 times
resize gun pic.jpg
The Rifle...
gun pic 2 resize.jpg
The valve parts... (The blowvalve position is only temporary. I will/would have placed it more in the trigger area.)
gun pic 3 resize.jpg
Here is a close up, side pic of the bolt.
gun pic 4 resize.jpg
Here is another view of the bolt

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Unread postAuthor: sputnick » Fri Jan 23, 2009 11:59 pm

Could you give an inside diagram of it, because I canot see how the barrel lines up inside the gun, and that is really the critical info here :lol:
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Unread postAuthor: Gippeto » Sat Jan 24, 2009 12:15 am

I use a modded blowgun to pilot a 2" piston valve, so I am inclined to look elsewhere for the problem.

How well does the piston actually fit? ie. If you were to remove the fitting on the back of the tee, and point the muzzle at the sky, would the piston fall out?

What does happen when you vent the pilot?

What is the surface area differential for your piston?

What is your pilot volume, and total piston travel?

And just because I've run into it before...Are you trying to vent your pilot through rolled up socks? :wink:
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Unread postAuthor: thedeathofall » Sat Jan 24, 2009 4:58 pm

@sputnick, It's your basic tee-piston valve

@Gippeto, This piston fits nicely, but isn't air tight. If it is pointed upright, it falls out. I still think that the valve cant handle the pilot volume. When the pilot is vented, the air just flows out the pilot. I haven't glued all the parts together. So it hasn't really been pressure tested yet. I have been trying to get the piston to seal the barrel. I finally just bought neoprene on Mc Master.

I'm not sure what you mean by the surface area differential.

The piston only travels about 1/4 - 3/8 of an inch. Most of the pilot volume is caused by the fittings.

HAHA I don't even know how that is possible.

Thanks guys for getting back to me on this. I've been working on this project for months and i really just want it to work. :lol: [/quote]
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Attachments
Piston daigram.jpg
This is a simple diagram of the valve.
Piston daigram.jpg (11.37 KiB) Viewed 422 times

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Unread postAuthor: Gippeto » Sun Jan 25, 2009 12:08 am

Give the piston a couple wraps of tape until it won't fall out (just), and try it again.

Piston area differential explained simply.

If you do not have enough of your piston face exposed to chamber pressure, you can have problems getting the piston to actuate.

ie. If you have a 1" barrel and a 1" piston, you have no surface area for the chamber pressure to act upon, and the piston will not actuate.
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Unread postAuthor: thedeathofall » Sun Jan 25, 2009 1:07 am

Okay, thanks, I'll see what I can do. It happens to have a 1/2 inch barrel, but if the tape won't work, I'll try cutting a new sealing face.
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Unread postAuthor: MrCrowley » Sun Jan 25, 2009 1:29 am

A 1" blowgun should be enough (but not the best) to pilot a 1" barrel sealing tee piston. Since pilot volume looks minimal and your piston's surface area looks like a good ratio from your rough diagram, the problem is down to the piston's fit inside the tee.

Since it's a solid rod, you could try tracing a circle around it and then making an o-ring groove with a hacksaw and file, this will work and guarantee a great seal for the piston. Tape may work, but I wouldn't recommend it. Instead I would recommend copious amounts of lubricant and maybe a resin that you could 'paint' on to the piston evenly with a very fine layer to give it some more thickness.

If you put too much on, just sand it down. Best choice would be to make an o-ring groove, and do the resin, but if it's a tight enough fit already, lubricant can help fill in the gap. Though it may need to be replaced every now and then. Which doesn't seem like too much of a problem for you.
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Unread postAuthor: thedeathofall » Sun Jan 25, 2009 1:20 pm

Well, I tried tape, it didn't work. I also cut a new sealing face. It is as big as I can make it and still get it to fit inside my T. It's still not working. I think I'll go with MrCrowley's idea. I just don't have the right size o-ring. Off to OSH I go...
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Unread postAuthor: Gippeto » Sun Jan 25, 2009 8:39 pm

MrC was leaning toward a more "permanent" fix, and has some very good suggestions.

I was leaning more toward identifying the problem/s first, using temporary fixes.(tape)

If you couldn't get it to at least work with the tape, the o-ring is not likely to help either.

Something else is amiss as well.

How true to scale is your drawing? Does the barrel actually protrude into the tee as far as shown?
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Unread postAuthor: MrCrowley » Sun Jan 25, 2009 9:03 pm

I was leaning more toward identifying the problem/s first, using temporary fixes.(tape)
That is a good point. I suppose I did rush ahead but I can't think of what else the problem could be judging if the drawing is accurate.
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Unread postAuthor: thedeathofall » Sun Jan 25, 2009 11:34 pm

After reviewing my drawing, and the weapon itself, I have come to the conclusion the I had a sucky memory at the time I made the drawing... :lol:

The drawing was a little out of scale. I am now thinking, Gippeto was right, the differential may still not be enough. I will go with the o-ring route, just to be sure, but I may have to move up to a bigger Tee to give me room to make the sealing face even bigger.
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Attachments
piston diagram resize.JPG
Updated piston diagram
piston diagram resize.JPG (8.53 KiB) Viewed 342 times

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Unread postAuthor: MrCrowley » Mon Jan 26, 2009 2:09 am

Even with that drawing, it looks like plenty enough. jackssmirkingrevenge has even got pistons working with almost no area exposed.
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Unread postAuthor: Pilgrimman » Mon Jan 26, 2009 3:29 am

Yeah I had a 1 1/4" piston in a 2" tee sealing a 1 1/2" barrel and it actuated fine even with virtually no surface area. Aslong as the flow rate around the piston to the pilot is not to high, it should actuate, short of an unforseen problem not yet described.
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